CHAPTER IV 

 THE AIR 



I. Atmospheric Pressure. Growth under decreased or increased atmospheric pres- 

 sure. Wieler's and Jaccard's experiments. Atmospheric pressure on high mountains. 

 3. The Air contained in Water. Sokibility, composition, and diffusion of air in water. 

 Devices for tlie absorption and transport of oxygen in aquatic plants. Aerenchyma and 

 other aerating tissues. Pneumatophores. G. Karsten's and GreshofiPs experiments. 

 3. The Wind. i. ]]'ind and Trce-groiuih. Mechanical influences. Drying influences, 

 njurious cftccts of the latter on tree-growth, ii. Wind and Reproduction. Anemo- 

 philous leaves. Their frequent presence in windy localities. Anemophilous devices 

 'or dispersal of seeds. Importance in relation to distribution at great distances apart. 

 Treub's observations at Krakatoa. 



This chapter will not deal with the endless and manifold relations 

 xtwcen vegetation and the atmosphere, but will treat only of some 

 jhenomena connected with the oecological character of certain formations 

 Df plants or with the distribution of species. 



I. ATMOSPHERIC PRESSURE. 



As Wieler and Jaccard have shown, the pressure within the stratum of the 

 itmosphere in which plants grow does not at all correspond to the absolute 

 )ptinium pressure for the growth of plants. On the contrary, a diinimi- 

 'ion of the partial pressure of the oxygeii—iox the latter only, and not the 

 otal atmospheric pressure, comes into question — occasions an acceleration 

 n growth until a certain low pressure is attained, which is constant for 

 ach species, and beyond which any further diminution in pressure causes 

 I retardation in the rate of growth. We find this absolute optimum 

 itmospheric pressure for growth to be in the case of Helianthus annuus 

 ibout 100 mm., but for Vicia Faba about 200 mm. Again, an increase in 

 tmosplieric pressure above 760 mm. (or the corresponding pressure of 

 )xygen) up to about ai atmospheres occasions a retardation, but after that 

 m acceleration in growth. There are therefore for growth two absolute 

 ptima of atmospheric pressure, both of which differ considerably from the 

 )ressures that prevail in the inhabited stratum of air, the one being at a far 

 ower, and the other at a far higher ox}-gen-prcssure. 



According to Jaccard a decrease in the pressure of oxygen occasions not 

 )nly more rapid growth, but also richer branching in the axes and roots, as 

 veil as an increase in the size of the leaves. Jaccard's following tabular 



